1. Field of the Invention
The present invention relates to a spark plug, for, for example, of an internal combustion engine, especially having an improved structure of a front (tip) end portion of a center electrode thereof.
2. Relevant Art
A spark plug generally comprises a main metal fitting having a cylindrical or tubular shape, an insulator or insulating member accommodated and held in the main metal fitting, a central electrode disposed and held in the insulator in a manner that the front (tip) end portion of the center electrode is exposed outside the insulator, and a grounding electrode (earth electrode) mounted to the main metal fitting so as to oppose to the tip end portion of the center electrode with a discharge gap therebetween.
In the spark plug having such structure, a spark is generated in the discharge gap by applying a voltage. At this spark generation, when a high discharge voltage is applied, the electrode will be consumed earlier, and in order to suppress such earlier consumption of the electrode, it is effective to lower the discharge voltage.
Moreover, in order to effectively utilize a limited combustion space of an engine, recently, it has been required to make compact the spark plug and an ignition coil for applying a voltage to the spark plug. However, in order to make compact the structure of the spark plug, it is generally required to make thin the thickness of the insulator surrounding the center electrode, which will result in that it is difficult for the insulator to withstand against the high discharge voltage in its shape. In this meaning, it becomes necessary to suppress or lower the discharge voltage at the time of ignition.
In conventional art, there has been known a method, as a method of lowering the discharge voltage, in which a columnar tip constituting the tip end portion of the center electrode is formed so as to have a small diameter to thereby concentrate electric field on this fine tip end portion.
It is, however, difficult to obtain an effective lowering of the discharge voltage by considerably making thin the tip end portion of the center electrode, and for example, in a case of the tip end portion having diameter of less than 0.4 mm, no advantageous effect could not be achieved. Accordingly, in the conventional structure, there exists a critical limit to make fine the diameter of the tip end portion.
Furthermore, in the conventional art, there has been further provided a method of reducing the discharge voltage, such as disclosed in Japanese Patent Laid-open (KOKAI) Publication No. HEI 1-109675, in which the tip end portion of the center electrode is formed so as to have a fine diameter and a tapered surface is formed between this fine diameter portion and the center electrode body portion. However, this tapered surface provides a recessed shape towards an axis of the center electrode and not linear shape.
However, in the structure of the tip end portion of the center electrode having the tapered surface as mentioned above, it is difficult to form or work the recessed portion with uniform dimension, thus being also disadvantageous.
In addition, there is further proposed a structure of the tip end portion of the center electrode having such tapered recessed portion in which a length between the small diameter portion of the tip end portion and the connection portion of the tapered surface is made larger. According to this structure, the discharge voltage may be further lowered.
In such structure, however, in a case where this length is made too long of, for example, more than 1.3 mm, a temperature of the tip end portion becomes considerably high and the electrode is hence consumed earlier, thus being also inconvenient.